This invention relates to a battery and a method of fabricating thereof. More particularly, it relates to a battery which secures safety while retaining high discharge capacity by suppressing a temperature rise due to a short-circuit, etc. and a method of fabricating thereof.
In recent years, with the development of electronic equipment, batteries used therein as a power source have increasingly gained in capacity and output density. A lithium ion secondary battery is attracting attention as a battery fulfilling these requirements. A lithium ion secondary battery has an advantage of high energy density but requires sufficient measures for safety because of use of a nonaqueous electrolytic solution.
Conventionally proposed safety measures include a safety valve which relieves an increased inner pressure and a PTC element which increases resistivity on heat generation due to an external short-circuit to shut off the electric current. For example, incorporation of a safety valve and a PTC element into the cap of a positive electrode of a cylindrical battery is known as disclosed in JP-A-4-328278. However, on the safety valve""s working, moisture in the air enters the inside of the battery, which can induce an exothermic reaction in case lithium exists in the negative electrode.
On the other hand, a PTC element, which cuts off the external circuit involving a short-circuit, exerts no bad influence on operating. The PTC element can be designed to operate when the battery temperature rises to, for example, 90xc2x0 C. or higher due to an external short-circuit so as to be the first safety element to operate in case of abnormality.
FIG. 9 shows an example of conventional lithium secondary batteries with a PTC element having the above-described constitution. Having the structure illustrated in the Figure, conventional lithium secondary batteries involve the following problems. As shown in the Figure, a conventional lithium secondary battery has a PTC element 14 in the cap part (the part having a safety valve 16) fitted on the top of a case 17. In case where a short-circuit occurs inside the battery, i.e., in the electrode 15 deeper than a lead 13, and the battery temperature rises due to the short-circuit, the PTC element 14 is incapable of suppressing the increase in the short-circuit current.
When a short-circuit occurs in the inside of a lithium secondary battery to raise temperature, a separator made of polyethylene or polypropylene interposed between a positive electrode and a negative electrode is expected to soften or melt to clog the pores of the separator, whereby the separator would exude the nonaqueous electrolytic solution contained therein or seal the nonaqueous electrolytic solution within itself to reduce its ion conductivity thereby to diminish the short-circuit current. However, the part of the separator distant from the heat generating part does not always melt. Besides, in case temperature rises, it is likely that the separator melts and flows to lose its function of electric insulation between positive and negative electrodes, which can lead to a short-circuit.
In particular, in the case of a lithium ion secondary battery, the negative electrode is prepared by coating a substrate functioning as a current collector, such as copper foil, with a slurry comprising a negative electrode active material such as graphite, a binder such as polyvinylidene fluoride (PVDF), and a solvent, and drying the coating layer to form a film. The positive electrode is similarly prepared in a film format on a substrate functioning as a current collector, such as aluminum foil, comprising a positive electrode active material, such as LiCoO2, a binder, and a conducting agent. The conducting agent is to enhance electron conductivity of the positive electrode in case where the active material has poor electron conductivity. The conducting agent to be used includes carbon black (e.g., acetylene black) and graphite (e.g., artificial graphite KS-6, produced by Lonza).
When the temperature of such a battery increases to or above the temperature at which the separator melts and flows due to, e.g., an internal short-circuit, a large short-circuit current flows between the positive and negative electrodes at the part where the separator flows as mentioned above. It follows that the battery temperature further increases by heat generation, which can result in a further increase of the short-circuit current.
The invention has been made in order to solve the above-described problems. An object of the invention is to provide a battery which secures safety while retaining high discharge capacity and a method of fabricating thereof by using an electrode capable of suppressing an increase of a short-circuit current.
A first battery according to the invention is a battery having an active material layer comprising an active material and an electron conductive material in contact with the active material and an electrolyte layer joined to the active material layer, which is characterized in that the electron conductive material contains a conductive filler and a resin and is constituted so as to increase its resistivity with a rise in temperature, and the average particle size of the active material is 0.1 to 20 times that of the electron conductive material. According to this aspect, there is obtained a high-performance and highly safe battery which is capable of suppressing an increase in current flowing through the electrode in case of a temperature rise.
A second battery according to the invention is the above-described first battery, characterized in that the resin comprises a crystalline resin. The resin comprising a crystalline resin according to this aspect, the rate of increase in resistivity (i.e., the rate of change in resistivity) with temperature can be heightened so that there is provided a battery which can quickly suppress an increase in current flowing through the electrode in case of a temperature rise.
A third battery according to the invention is the above-described first battery, characterized in that the electron conductive material has an average particle size ranging from 1 xcexcm to 10 xcexcm. The particle size of the electron conductive material ranging from 1 to 10 xcexcm, the electrode shows an increased rate of change in resistivity in case of a temperature rise while having a reduced resistivity in its normal state so that a battery having the electrode can have an increased discharge capacity.
A fourth battery according to the invention is the above-described first battery, characterized in that the weight ratio of the electron conductive material to the active material ranges from 0.06 to 0.15. With the electron conductive material to active material weight ratio ranging from 0.06 to 0.15, it is possible to desirably adjust the resistivity of the electrode before the rate of resistivity change increases and the discharge capacity.
A fifth battery according to the invention is the above-described first battery, characterized in that the resin has a melting point ranging from 90xc2x0 C. to 160xc2x0 C. According to this aspect, since a resin having a melting point of 90 to 160xc2x0 C. is used, the electron conductive material shows an increased rate of resistivity change at around a predetermined temperature within a range of from 90 to 160xc2x0 C. thereby achieving security consistent with battery characteristics.
A sixth battery according to the invention is the above-described first battery, characterized in that the proportion of the conductive filler in the electron conductive material is 40 to 70 parts by weight. The conductive filler content in the electron conductive material ranging from 40 to 70 parts by weight, the electrode shows an increased rate of change in resistivity in case of a temperature rise while having a reduced resistivity in its normal state, and the battery has an increased discharge capacity.
A seventh battery according to the invention is the above-described first battery, characterized in that the conductive filler is a carbon material or a conductive non-oxide. Containing a carbon material or a conductive non-oxide as a conductive filler, the electrode has enhanced conductivity.
An eighth battery according to the invention is the above-described first battery, characterized in that the active material layer contains a conducting agent. Since the electrode contains a conducting agent, the resistivity of the electrode can be properly adjusted even in using an electron conductive material having low electron conductivity.
A first method of fabricating a battery according to the invention is characterized by comprising the steps of:
(a) pulverizing an electron conductive material containing a conductive filler and a resin to prepare fine particles of the electron conductive material the average particle size of which is 0.05 to 10 times that of an active material,
(b) dispersing the fine particles of the electron conductive material and the active material in a dispersing medium to prepare an active material paste,
(c) pressing the active material paste having being dried at a prescribed temperature under a prescribed pressure to form an electrode, and
(d) laying the resulting electrode and an electrolyte layer together.
Comprising the steps (a) to (d), the process provides a battery which retains a high discharge capacity and is capable of suppressing an increase in current flowing through the electrode in case of a temperature rise.
A second method of fabricating a battery according to the invention is the first process which is characterized in that the resin comprises a crystalline resin. According to this aspect, since the resin comprises a crystalline resin, the rate of increase in resistivity (i.e., the rate of change in resistivity) with temperature can be heightened so that there is provided a battery which can quickly suppress an increase in current flowing through the electrode in case of a temperature rise.